This application is a continuation-in-part of copending U.S. Ser. No. 07/547,419, filed Jul. 2, 1990, and now U.S. Pat. No. 5,005,407 which is a file wrapper continuing application of U.S. Ser. No. 07/256,605, filed Oct. 11, 1988, and now abandoned without prejudice.
This invention relates generally to shielding for a sensing system for determining or measuring the level (i.e., quantity) of a stored medium in a storage container. More particularly, this invention relates to an improved shielded level sensing arrangement for relatively more reliably sensing (and optionally for displaying at a relatively remote location) the level of a stored medium in a non-conducting container.
Level measuring systems and indicators therefor typically have been limited to arrangements which include a sensing element or apparatus emersed in the medium within the container. Many level gauges require that the subject stored material (i.e., the material or medium the level of which is to be detected) be in electrical contact with the gauge, while some others require that the stored material or medium be itself electrically conductive.
In the prior art, U.S. Pat. No. 2,868,015 discloses a level indicator for conductive liquids utilizing a plurality of vertically stacked capacitive elements arranged in a tank. The inherent drawback generally in such systems is that they are operable only with electrically conductive filler fluids which replace the fuel actually being measured, and they are potentially dangerous if the medium being measured comprise combustible fluids, such as gasoline, or the like. Also, the capacitive unit disclosed in such patent generally must be emersed within the fluid in the tank, and is apparently therefore not adaptable to be utilized exteriorly of the tank.
The liquid level measuring device disclosed in U.S. Pat. No. 2,751,531 also requires the sensing condenser to be emersed in the tank containing the liquid the level of which is to be sensed. The complex structure of the capacitive unit is such that it is completely incapable of use outside of the tank.
U.S. Pat. Nos. 3,010,320; 3,025,464; 3,321,971; 3,862,571; and 3,935,739 also generally disclose level measuring devices for liquids which require electrical contact with the liquids being measured. Such devices generally suffer from the same drawbacks noted above with reference to the other patents requiring electrical contact between the mediums being measured and the various sensing probe members.
The use of an externally applied sensing device is disclosed for example in U.S. Pat. No. 4,165,641. That device is in the form of an inductance coil or rather a plurality of coils wound on a coil former for holding and shaping the coils. As the level of the liquid in the container rises or falls it comes into close proximity to one of the inductive coils. The electrical conduction of the liquid changes the Q of the coil thereby quenching oscillation in a detector circuit. An important ingredient in the operation of this level sensing device is that the fluid (the level of which is being detected) should be conductive to some extent. Therefore, it is only operable with limited types of material, such as at least partially conductive materials. The use of inductive coils also limits the application of such measuring means to liquid containers having coil carrying tubes.
Sensing structures are also subject to adverse external influences. For example, electrostatic interference can be introduced by the proximity of a human hand and/or other objects adjacent the sensors, with the result being false indications of sensed medium levels or falsely indicated changes in such levels. In other words, the potential for adverse external electrostatic influences gives rise generally to the potential for relatively less reliable overall sensing system performance.
In general, one previously known concept is referred to as a Faraday shield (or cage) and comprises wire mesh or a series of parallel wires connected at one end to ground, so as to compose an electrostatic shield.